Stopper Rod for Delivering Gas Into a Molten Metal

ABSTRACT

The invention relates to a stopper rod adapted to deliver gas during pouring of molten metal comprising a stopper body having an internal chamber and a gas discharge port, a bore connecting the internal chamber to the gas discharge port, the internal chamber and the bore defining a gas passageway. According to the invention, the walls of the gas passageway are provided with a layer of a material which will not produce carbon monoxide at the temperature of use. The stopper rod of the invention does not contaminate the gas passing therethrough.

This invention relates to a mono-block stopper rod used to control the flow of molten metal from a discharge nozzle in a holding vessel during metal teeming.

In continuous casting processes, the use of gases injected down the stopper has been shown to have significant benefits on the quality of metal being cast. For example, inert gases such as argon or nitrogen can be injected to reduce the problems due to alumina build-up and clogging or to assist in removing solidification products from the vicinity of the discharge nozzle. Reactive gases may also be employed when the melt composition needs modifying. Conventionally, the stopper—generally made from an alumina carbon refractory composition—is provided with an internal chamber connected to gas supply means on the one end and to a gas discharge port at the other end.

Various systems have been developed to ensure an accurately measured flow of gas is supplied to the stopper. Problems have been encountered with sealing such systems and ensuring that the gas follows its intended path and is not wasted. Stoppers which have proved to be successful in meeting many of these requirements are disclosed in EP-A2-358,535, WO-A1-00/30785 and WO-A1-00/30786 and more recently in WO-A1-02/100579.

Pursuing its development work in this field, the applicant has now realized that the gas injected into the molten metal through the stopper could be contaminated when passing through the stopper.

In particular, it is suspected that the carbon present in the composition constituting the stopper body could reduce some of the metal oxides also present in the composition; this reduction is accompanied by the generation of carbon monoxide. The carbon monoxide injected into the molten metal will in turn oxidize the aluminum which has been added to calm the steel producing thereby important quantities of alumina contributing thus to the alumina build-up and clogging.

It would therefore be advantageous to provide a stopper rod which would not contaminate the gas passing therethrough.

According to the invention, this problem is solved for a stopper rod adapted to deliver gas during pouring of molten metal comprising a stopper body having an internal chamber and a gas discharge port, a bore connecting the internal chamber to the gas discharge port, the internal chamber and the bore defining a gas passageway. According to the invention, the walls of the gas passageway are provided with a layer of a material which will not produce carbon monoxide at the temperature of use.

It must be noted that it is, to the applicant's knowledge, the first time that it is proposed to provide a refractory article with such a layer on a portion of said article that will never contact the molten metal. On the contrary, in the prior art as exemplified for example by the document U.S. Pat. No. 5,691,061 or U.S. Pat. No. 5,681,499, such a layer is only provided on portions that are always in contact with the metal.

The layer can be formed as a coating applied on the walls of the gas passageway after the manufacture of the stopper rod. Such a coating can be applied by spraying a liquid, wet or semi-wet composition or simply by filling the inner chamber with the appropriate composition. Once the coating is dried, the stopper rod can then be cured. Alternatively, the curing of the stopper can take place before the coating step. Advantageously, the layer is a liner pressed simultaneously with the stopper rod body. In this case, it is indeed possible to reduce the number of manufacturing steps. According to another variant, the layer extends through essentially the whole thickness of the walls of the gas passageway.

The material constituting the a layer which will not produce carbon monoxide at the temperature of use can be selected from three different categories of materials:

a) materials which do not contain carbon;

b) materials essentially constituted of non reducible refractory oxides; or

c) materials comprising elements which will react with the generated carbon monoxide.

Preferably, the selected material will present two or three of the above properties.

Examples of suitable material of the first category are silica (for example vitreous silica), alumina, mullite or magnesia based material (spinel). In certain cases, these materials can however be somehow difficult to apply as a liner or coating (the lack of carbon in the layer can cause some thermal shock problems) and do not constitute the preferred embodiment of the invention.

Suitable materials of the second category are for example pure alumina carbon compositions. In particular, these compositions should contain very low amount of silica or of conventional impurities which are usually found in silica (sodium or potassium oxide). In particular, the silica and its conventional impurities should be kept under 2 wt. %, preferably under 1 wt. %.

Suitable materials of the third category comprises for example free metal able to combine with carbon monoxide to form a metal oxide and free carbon. Silicon and aluminum are suitable for this application. These materials can also or alternatively comprise carbides or nitrides able to react with carbon monoxide (for example silicon or boron carbides).

Preferably the selected material will belong to the second or third categories, even preferably, it will belong to the second and third category.

A suitable material constituting the layer which will not produce carbon monoxide at the temperature of use can comprise 60 to 88 wt. % of alumina, 10 to 20 wt. % graphite and 2 to 10 wt. % of silicon carbide. Such a material is essentially constituted of non-oxides species or non-reducible oxides and comprises silicon carbide which can react with the carbon monoxide if some is generated in working condition.

The invention will now be described with reference to the attached drawing in which

FIG. 1 shows a cross-section of a stopper rod according to the invention.

Reference 1 depicts respectively the internal chamber and a gas discharge port of the stopper rod. A bore 3 connects the internal chamber 1 to the gas discharge port 2. The bore 3 and the internal chamber 1 define a passageway for the gas. FIG. 1 represents an embodiment wherein the layer 4 has been copressed as a liner with the stopper body. Very conveniently, the liner 4 can be constituted of several pre-formed tubular portions (41, 42, 43) which are all copressed with the stopper body. A portion of the metal rod 5 connecting the stopper rod to the rigging device (not shown) is depicted in working position. Preferably, the metal rod 5 extends beyond the highest point of the layer 4. Even more preferably, a sealing gasket 6 is inserted around the lower end of the metal rod 5. 

1-9. (canceled)
 10. A stopper rod adapted to deliver gas during pouring of molten metal comprising a stopper body including walls defining a gas passageway fluidly connected to a gas discharge port, the walls comprising a layer that will not produce carbon monoxide at use temperature.
 11. The stopper rod of claim 10, wherein the layer comprises a liner copressed with the body.
 12. The stopper rod of claim 10, wherein the layer comprises a coating applied on the walls of the gas passageway.
 13. The stopper rod of claim 10, wherein the walls include a thickness and the layer extends through essentially the thickness of the walls.
 14. The stopper rod of claim 1, wherein the layer extends beyond a lowest point reached by a metal rod that attaches the stopper to a rigging device.
 15. The stopper rod of claim 1, wherein the layer comprises a material selected from a group consisting of materials that do not contain carbon; materials consisting essentially of non-reducible refractory oxides; materials that react with carbon monoxide, and mixtures thereof.
 16. The stopper rod of claim 1, wherein the layer comprises 60-88 wt. % alumina, 10-20 wt. % graphite, and 2-10 wt. % silicon carbide.
 17. A stopper rod assembly adapted to deliver gas during pouring of molten metal, the assembly comprising a stopper rod including a stopper body and walls defining a gas passageway fluidly connected to a gas discharge port, the walls comprising a layer that will not produce carbon monoxide at use temperature; a metal rod attaching the stopper rod to a rigging device, the metal rod extending into the gas passageway beyond a highest point of the layer.
 18. The assembly of claim 17, wherein a sealing gasket extends around a lower end of the metal rod.
 19. A stopper rod adapted to deliver gas during pouring of molten metal comprising a stopper body having an internal chamber and a gas discharge port, a bore fluidly connecting the internal chamber to the gas discharge port, the internal chamber and the bore defining a gas passageway including walls comprising a layer that will not produce carbon monoxide at the temperature of use. 